The subject matter of the present invention is a process for the cleavage of chlorosiloxanes at elevated temperature, in which silicon tetrachloride forms as the cleavage product, and, in some case, trichlorosilane. The chlorosiloxanes to be cleaved occur mainly as by-products in the production of tri- and tetrachlorosilane, so that the claimed process contributes toward increasing the yield of chlorosilanes in the production of chlorosilanes, and toward reducing, or completely eliminating, the amount of by-products.
In the large-scale technical production of silicon tetrachloride in reactors by the reaction of lump silicon or siliceous material, such as ferrosilicon for example, hexachlorodisiloxane, among other substances, occurs as by-product. If the reaction is performed with hydrogen chloride, a chlorosiloxane mixture is obtained which consists mainly of hexachlorodisiloxane and pentachlorodisiloxane.
These chlorodisiloxanes are formed by the reaction of chlorosilanes with water, which enters the production process as moisture with moist raw materials such as chlorine, hydrogen chloride, or metallic silicon or, in discontinuous processes, gains entry into the production apparatus with humid air when the reactor is charged and the ash is cleaned out.
Other by-products in the reaction of siliceous material with chlorine or hydrogen chloride are metal chlorides and ash. The metal chlorides are formed by the reaction of metals contained in the raw silicon. The content of foreign metals in the raw silicon usually amounts to 2 to 10 wt.-%; iron and aluminum are the principal such metals.
Usually the silicon tetrachloride and/or trichlorosilane main product is separated from the above by-products by distillation. This results in a mixture of chlorodisiloxanes, metal chlorides and ash, which must be disposed of along with a small amount of silicon tetrachloride.
It is known to dispose of the by-products by hydrolyzing all of them, the disiloxanes being converted to silicic acid and the metal chlorides to metal oxides or oxide hydrates, and hydrogen chloride is released, in the form of hydrochloric acid in some cases. The disadvantage of this procedure consists on the one hand in a loss of raw material, i.e., a reduced yield of the overall process, and on the other hand in the fact that a silica solution containing hydrochloric acid is produced, which again must be disposed of.
It is furthermore also known that hexachlorodisiloxane can be cleaved to silicon tetrachloride and octachlorotrisiloxane at temperatures between 70.degree. and 137.degree. C. (C.A. Vol. 94, page 692, 40670 r). The technical practice of this procedure faces the obstacle that only a small part of the silicon reacts to produce a salable product, while most of the silicon is converted to a product which cannot be sold and presents the same disposal problems.
The problem was therefore to find a method of cleaving chlorosiloxanes such that largely salable end products will be formed, which present no additional disposable problems and most of which can be utilized economically.